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In Gambit 2.1, if one assigns boundary conditions for the fluid zone, which is meshed using hex core mesh, Gambit is unable to write the correct mesh file. You will get Null Domain Pointer error when reading this mesh into Fluent. This limitation is only for fluid zone, not boundary face. It has been fixed in Gambit 2.2.The workaround to get rid of this problem in Gambit 2.1 is to remove any boundary condition specification for the hex core volume in Gambit 2.1 and write the mesh file again. This mesh file, in which there is no bc specification for the hex core fluid zone reads well in Fluent.